<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <title>Document</title>
  <style>
    * {
      margin:0;
      padding:0;
    }
    body{
        height: 1800px;
    }
    img{
      display:block;
      border:none;
    }
    /* 原图图片所在区域 */
    #box{
      position: relative;
      margin-top:20px;
      margin-left:20px;
      width:350px;
      height:350px;
      box-shadow:3px 3px 10px 0 #111111;

    }
    /* 原图图片大小 */
    #box img{
      width:100%;
      height:100%;

    }
    /* 鼠标所在区域 */
    #mark{
      display:none;
      position:absolute;
      top:0;
      left:0;
      width:175px;
      height:175px;
      background:#000;
      opacity: 0.5;
      filter:alpha(opacity=50);
      cursor:move;

    }
    /* 放大图片展示区域 */
    #boxRight{
      display:none;
      position:absolute;
      top:20px;
      left:380px;
      width:350px;
      height:350px;
      overflow:hidden;
    }
    /*我们右侧的图片的大小是需要严格计算的:
      mark的width是box的width的一半，那么我们的大图宽度也应该是小图的二倍
    */
    #boxRight img{
      position:absolute;
      top:0;
      left:0;
      width:200%;
      height:200%;
    }
  </style>
</head>
<body>
    <!-- <div style="width:200px;height: 200px;border:1px solid red;"></div> -->
  <div id='box'>
    <img src="../images/chu_liuxiang.png" alt="">
    <div id='mark'></div>
  </div>
  <div id='boxRight'>
    <img src="../images/chu_liuxiang.png" alt="">
  </div>
  <script>
    //放大镜的原理: 我们的mark横向是box的一半，纵向也是box的一半，那么右侧的大图横向（纵向）应该是左侧小图的2倍
    var box = document.getElementById('box');
    var mark = document.getElementById('mark');
    var boxRight = document.getElementById('boxRight');
    console.log(document.documentElement.scrollTop || document.body.scrollTop) //  页面整体滚动的Y距离
    console.log(box.offsetTop)//  当前对象(不包括border)到其上级层顶部的距离，此处指box到body主体的距离（跟滚动无关）
    //设置mark这个盒子
    function setPosition(e){
      var top = e.clientY - box.offsetTop - (mark.offsetHeight/2);
      var left = e.clientX - box.offsetLeft - (mark.offsetWidth/2);

      //边界判断
      var tempL = 0,tempT = 0;
      var minL = 0,minT = 0,maxL = box.offsetWidth - mark.offsetWidth,maxT = box.offsetHeight - mark.offsetHeight ;

      if(left<minL){
        mark.style.left = minL + "px";
        tempL = minL;
      }else if(left>maxL){
        mark.style.left = maxL + "px";
        tempL = maxL;
      }else{
        mark.style.left = left + "px";
        tempL = left;
      }
      if(top<minT){
        mark.style.top = minT + "px";
        tempT = minT;
      }else if(top>maxT){
        mark.style.top = maxT + "px";
        tempT = maxT;
      }else{
        mark.style.top = top + "px";
        tempT = top;
      }
      //右侧图片跟着运动:左侧移动多少，右侧跟着移动他的2倍即可
      var oImg = boxRight.getElementsByTagName("img")[0];
      oImg.style.left = -tempL*2 + "px";
      oImg.style.top = -tempT*2 + "px";

    }
    box.onmouseenter = function(e){
      e = e || window.event;
      
      mark.style.display = "block";
      setPosition(e);
      boxRight.style.display = "block";

    }
    box.onmousemove = function(e){
      e = e || window.event;
      setPosition(e);

    }
    box.onmouseleave = function(e){
      e = e || window.event;
      mark.style.display = "none";
      boxRight.style.display = "none";
      
    }
  </script>
</body>
</html>
